Adjustable lenses may mean no more bifocals

Along with thinning hair,
a few more wrinkles and some added pounds, by age 50 nearly everyone notices that
print seems harder to read. It’s the result of presbyopia, an age-related
inability of the eye to focus on near objects.

Solutions to the problem have included reading
glasses as well as bifocals, trifocals and progressive lenses. However, all multifocal
lenses have a limited field of view, which makes some reading tasks difficult, and
they sometimes cause dizziness and discomfort.

Now a team from the University of Arizona
in Tucson and Georgia Institute of Technology in Atlanta has demonstrated a lens
that changes focusing power on demand while providing power-failure-safe operation
and avoiding the problems of multifocals. “Using the switchable lens, the
entire lens aperture is used for one vision task,” said Guoqiang Li, an assistant
research professor of optical sciences at Arizona. Nasser Peyghambarian, a professor
of optical sciences and materials science at Arizona, was the principal investigator.

In building switchable lenses with adjustable focus, researchers
constructed optics comparable to high-quality reading glasses. The unwrapped
phase map of the 10-mm aperture lens is on the top, and the bottom map
shows the difference between the measured wavefront and a best-fit spherical
wave and tilt. This shows there are few higher-order aberrations. Reprinted
with permission of PNAS.
The lenses consist of a 5-μm-thick
layer of liquid crystal sandwiched between two pieces of glass. In building these,
the researchers first fabricated eight concentric conducting rings on a glass substrate.
They covered everything with polyvinyl alcohol to act as a liquid crystal alignment
layer and attached the other substrate, leaving a small gap that they filled with
liquid crystal material. They sealed the two together and connected the drive electronics.

Switching states

When the investigators applied 2 V, the liquid
crystal material switched on, changing the effective refractive index in the eight
subzones of the 10-mm-diameter lenses. This changed the lens from simply being a
passive device to one adding focusing power.

Because the material was so thin, transmissivity
was high — about 85 percent with the voltage off and nearly as much with the
voltage on. Switching from one state to another took less than a second. With no
voltage applied, the lenses appeared to be plain glass, which Li noted is a valuable
characteristic for a variety of common tasks. “Power-failure-safe operation
is very important for driving,” he said.

After characterizing the lenses for
various optical parameters, the researchers tested them with a model of a human
eye constructed from a set of lenses, a filter and a sensor. They placed an object
at a distance of 30 cm in front of the model eye and captured the resulting blurry
image.

They then turned on their lens and
brought the object into sharp focus. These findings have been confirmed by some
clinical studies, which have had promising results. According to Li and Peyghambarian,
a company is working to commercialize the prototype used in those studies.

As for the future, the technology could
create lenses that allow multiple focal lengths to handle near, intermediate and
far vision tasks. The result could be glasses that adjust through the use of a distance-determining
near-infrared sensor and without requiring intervention by the wearer.

“The electronics control system decides the needed
focusing power and the corresponding voltages to apply,” Li said.